High-speed optical guided-wave switches and modulators are promising components for present and future applications in lightwave communication and measurement systems, or more generally for optical signal processing purposes. Integrated electro-optic modulators with bandwidth extending over a few gigahertz and low drive voltages are now commercially available. Due to their potential applications, bandwidth characterization is an important feature of the metrology of these devices and is often limited by the finite response time of the detection system. In this paper the bandwidth limitations of the electro-optic modulators are discussed and the authors report two techniques for bandwidth measurements, which have the significant advantage of using only low-frequency detection. This was made possible by processing the lightwave directly, using either a sampling technique with a narrow optical pulse as a probe signal or a direct optical spectrum analysis of the modulated signal.